The deformability of erythrocytes is also a factor in shear thinning, especially when the hematocrit is high. The mean diameter of human red blood cells is approximately 7 µm, but they are able to pass through openings with a diameter of only 3 µm. As blood with densely packed erythrocytes flows at progressively greater rates, the erythrocytes become more and more deformed. Such deformation diminishes the apparent viscosity of blood. The flexibility of human erythrocytes is enhanced as the concentration of fibrinogen in plasma increases (
Fig. 17.10 ). If the red blood cells become hardened, as they are in certain spherocytic anemias, shear thinning may diminish.
The Arterial System
The systemic and pulmonary arterial systems distribute blood to the capillary beds throughout the body. The arterioles are high-resistance vessels of this system that regulate the distribution of flow to the various capillary beds. The aorta, the pulmonary artery, and their major branches have a large amount of elastin in their walls, which makes these vessels highly distensible (i.e., compliant). This distensibility serves to dampen the pulsatile nature of blood flow that results as the heart pumps blood intermittently. When blood is ejected from the ventricles during systole, these vessels distend, and during diastole, they recoil and propel the blood forward (Fig. 17.11 ). Thus the intermittent output of the heart is converted to a steady flow through the capillaries.
The elastic nature of the large arteries also reduces the work of the heart. If these arteries were rigid rather than compliant, the pressure would rise dramatically during systole. This increased pressure would require the ventricles to pump against a large load (i.e., afterload) and thus increase the work of the heart. Instead, as blood is ejected into these vessels, they distend, and the resultant increase in systolic pressure, and thus the work of the heart, are reduced.
Aspeopleage,theelastincontentofthelargearteriesisreducedandreplacedbycollagen.Thisreducesarterialcompliance(
Fig.17.12 ).Thuswithage,systolicpressureincreases,asdoesthedifferencebetweensystolicanddiastolicbloodpressure,calledthepulse pressure (describedinthenextsection).
Determinants of Arterial Blood Pressure
Arterial blood pressure is routinely measured in patients, and it provides a useful estimate of their cardiovascular status. Arterial pressure can be defined as mean arterial pressure (Pa), which is the pressure averaged over time, and as systolic (maximal) and diastolic (minimal) arterial pressure within the cardiac cycle (
Fig. 17.13 ). The difference between systolic and diastolic pressure is termed pulse pressure.
The determinants of arterial blood pressure are arbitrarily divided into “physical” and “physiological” factors. The two
Systole Arterial blood flows through the
Diastole Arterial blood continues to flow through capillaries throughout systole.
the capillaries throughout diastole.
A When the arteries are normally compliant, a B During ventricular diastole the previously substantial fraction of the stroke volume is stretched arteries recoil. The volume of blood stored in the arteries during ventricular systole. that is displaced by the recoil furnishes The arterial walls are stretched. continuous capillary flow throughout diastole.
Left atrium Left atrium Left ventricle Left ventricle Aorta Aorta Capillaries Capillaries Systole A volume of blood equal to the entire stroke volume must flow through the capillaries during systole. Diastole Flow through the capillaries ceases during diastole.
C When the arteries are rigid, virtually none D Rigid arteries cannot recoil appreciably during of the stroke volume can be stored in the diastole. arteries.